The present invention relates to rotary type atomizers for applying paint and other materials in liquid atomized form, and more particularlv relates to a rotary atomizer adapted for electrostatic paint spraying.
The use of rotary atomizers for applying paint to coating surfaces has been long known in the art. These devices typically operate by rotating a disc or cup-shaped bell at high speed, and by applying a metered flow of liquid paint to the surface of the disc or bell as it is rotating. Centrifugal forces cause the paint supplied to the surface of the disc or bell to become hurled from its edge in droplets, which droplets are then directed toward a surface to be coated.
Rotary atomizers have also been used in conjunction with electrostatic forces for the application of paint, either by placing the rotary atomizer in a highly charged electrostatic field so as to induce the atomized paint particles to accept electrostatic charges and thereby become attracted to a grounded workpiece, or by directly voltage charging the rotary atomizer and thereby causing the paint droplets to become electrostatically charged as they are emitted from the edge of the rotating disc or bell.
In applications where the atomizer itself is voltage charged, the working voltages are typically in the range of 50-150 kilovolts (kv), and therefore a high degree of care must be taken to properly protect the charged components from inadvertent contact with people or nearby objects. Such systems are typically shielded from any possible contact by means of fences, booths, or other similar shielding constructions.
The hazards of prior art electrostatic rotary atomizers have limited the type and scope of applications in which such systems may be used. For example, such systems can only be used in applications wherein sufficient spatial separation is available to provide for relative isolation of the voltage charged rotary atomizer devices, and where a high degree of control can be maintained over the spacing between the atomizer device and articles moving past the device on a conveyor line. Extreme care is required in order to prevent accidental voltage discharges in solvent or other volatile atmospheres. Since prior art atomizers are constructed of metallic materials, or contain a high percentage of metallic materials in their construction, such atomizers inherently have a high value of electrical capacitance. When charged to the voltages associated normally with electrostatic paint spraying, these atomizers accumulate a very high amount of electrical energy in the form of capacitance stored energy. Therefore, if conditions occur wherein a voltage spark is generated, the capacitive energy stored in the atomizer itself will immediately dissipate through the spark, in sufficient energy quantities so as to cause ignition of volatile solvents and the like.
Some prior art rotary atomizers attempt to minimize this problem by applying a resistive coating to the surface of the atomizer disc or bell. This approach is described in U.S. Pat. No. 2,989,241, the substance of which is to incorporate an energy damping resistance between the high capacitance components of the rotary atomizer and the workpiece. This damping resistance effectively absorbs some of the electrical energy which would otherwise be dissipated in the form of a high energy spark, and thereby reduces the hazard of fire or explosion.
Despite the foregoing and other disadvantages which result from the use of prior art rotary atomizers, such devices have found widespread use in industry, for they do produce a finely atomized cloud or spray of paint and, as a result, produce a high quality coating on a workpiece. There is therefore a need to provide a rotary atomizer having the inherent advantages of high quality painting, but without the disadvantages associated with the various hazards.
It has been found that the quality of paint atomization is directly related to the rotational speed of the rotary atomizer, the higher the rotational speed the finer the atomization. Therefore, it is not unusual to find rotary atomizers which rotate in the range of 25,000-75,000 revolutions per minute (RPM), which itself produces additional problems. Conventional bearings are difficult and expensive to design to operate at high rotational speeds, and therefore it has been the practice in the industry to design rotary atomizers having various forms of air bearings to suspend the rotating members. Such air bearings have the advantage of providing long life of the rotating members, and therefore it is desirable to incorporate them into any rotary atomizer structure which is inherently less hazardous than heretofore known in the art.
The concept of utilizing an energy damping resistance between the capacitance charged components of an atomizer and the workpiece is an advantage which is also well-known in the art, at least in the form described hereinabove. Conventional automatic and manual spray guns utilize this same concept by placement of a physical resistance in a nonconductive spray gun body, which resistance is placed proximate the front end of the spray gun to accomplish the required electrical resistance damping. This approach in a design of conventional spray guns has greatly reduced the hazards associated with such guns, and it is desirable to incorporate such a concept into a rotary atomizer. However, prior art rotary atomizers which utilized such improvements as air bearing assemblies were required to be constructed of high precision metallic components, and such components inherently prevented the use of nonconductive bodies. It is therefore desirable to combine into a single rotary atomizer structure all of the advantages heretofore known with respect to conventional spray guns, air bearing technology, and rotary atomizer technology, so as to provide a new and improved rotary atomizer having all of the advantages in each field of technology.